The present invention relates to a catalyst and process for the oxidation of sulphur dioxide by air or oxygen-containing gases and to the recovery of the oxidation products thereof. In particular, the present invention relates to a catalyst and process for oxidation of sulphur dioxide at low temperatures in the presence of a catalyst and the recovery of oxidation products as sulphuric acid, or as reaction products of sulphur trioxide or sulphuric acid with organic compounds. In preferred embodiments, the present invention relates to the oxidation of sulphur dioxide from a source having a concentration of sulphur dioxide of between 0.01 and 15 vol. % SO2. Examples of such sources are flue gas, tail gases from sulphuric acid plants and off-gas from smelters. In addition, the sulphur dioxide may be obtained in plants for the manufacture of sulphuric acid or sulphur trioxide.
Sulphuric acid is the world""s most common industrial chemical. It is normally manufactured by either a) burning sulphur to form sulphur dioxide and converting the sulphur dioxide to sulphur trioxide over a multistage packed bed reactor using promoted vanadium pentoxide as catalyst, or b) oxidation of sulphur dioxide from waste gases in the same manner.
Oxidation of sulphur dioxide is a highly exothermic reaction, and the currently preferred catalysts are active only at high temperatures e.g. about 450-550xc2x0 C. The preferred catalysts are a eutectic mixture of vanadium pentoxide and potassium pyrosulphate supported on titanium dioxide, alumina, silica or minerals such as kieselguhr. Since the reaction is reversible and exothermic, the reactor usually consists of four trays in series that are operated adiabatically, in order to enhance the overall conversion. The reacting gas is always cooled before the last trays, and sometimes also after the intermediate trays. The catalyst layers are typically from about 15 to 50 cm deep, and consequently the cost of the catalyst is a large portion of the cost of the loaded reactor. For example, a plant that produces 1,000 tonnes of acid per day may contain 150,000 to 200,000 liters of catalyst.
The sulphur trioxide formed is dissolved in 98% sulphuric acid. If attempts are made to dissolve SO3 directly into water or into a weaker acid, the water vapour pressure causes the formation of an acid mist that is difficult to remove. The fortified H2SO4 that is obtained may then be diluted to the desired strength. In order to meet air pollution requirements, the gas leaving the scrubber must be further treated, which adds another expensive step.
Catalysts other than vanadium pentoxide are capable of oxidizing sulphur dioxide to sulphur trioxide. For example, platinum and other noble metals may be used. While such catalysts do tend to reduce the oxidation temperature, they also tend to be too expensive to be employed in commercial processes.
Activated carbon in the form of powder or pellets has shown activity that is comparable with that of vanadium oxide or platinum as catalyst for sulphur dioxide oxidation either in a xe2x80x9cdryxe2x80x9d or a xe2x80x9cwetxe2x80x9d process.
In a dry process, carbon is used to concentrate the SO2in the gas stream. Temperatures greater than 200xc2x0 C. must be used to remove sulphuric acid and/or SO3 from the carbon surface. In doing so, the carbon reduces both compounds to SO2 and is oxidized in turn. Thus, carbon is consumed in the dry process. When water is used to remove sulphuric acid and/or SO3 from the carbon surface i.e. a wet process, dilute sulphuric acid is the final product.
An improvement has been achieved by operating the reactor with periodic flushing of the carbon bed with a brief but high flow of water, as disclosed in Canadian Patent No. 1,229,843. Using this technique, the product has a moderate concentration but is still highly corrosive. Thus, the reactors used in the process must be capable of withstanding the effects of such acid e.g. stainless steel reactors must be used.
In an aspect of the present invention, it has now been found that an activated carbon that has been rendered hydrophobic or lyophobic with an appropriate polymer added to the carbon, shows an enhanced activity if the catalyst is flushed with water, sulphuric acid, an inert organic solvent, with a reactive organic compound or with a supercritical fluid that optionally contains inert organic solvent or reactive organic compound. In other aspects of the present invention, the catalyst may be used in a packed bed or supported on corrugated metallic or plastic screens or plates which make up structural packings with straight parallel channels or open cross-flow channels. In the latter cases, the hydrophobic or lyophobic polymer can serve as a binder to fix the fine powdered carbon on the support surface or may itself be the support medium.
In other aspects, it has been found that, if the oxidation is carried out with no water in the feed, the sulphur trioxide that is formed can be removed with a reactive organic substrate capable of forming organic acid sulphate or organic sulphonate compounds. If the oxidation is undertaken in the presence of a small excess of water, based on the stoichiometric amount required, highly concentrated sulphuric acid solutions can be obtained by flushing the catalyst with appropriate organic solvents. These solvents can contain some water. Thus, it has now been found that organic solvents and other organic compounds may be used in the stripping of the sulphur trioxide and sulphuric acid from the catalyst, and the resultant product is less corrosive to the materials used in the fabrication of the apparatus of the process.
Accordingly, one aspect of the present invention provides a process for the recovery of sulphur trioxide, solutions of sulphuric acid, or organic derivatives thereof, using organic compounds, comprising the steps of:
(a) passing a mixture of SO2 and an oxygen-containing gas over an activated carbon catalyst at a temperature of at least 15xc2x0 C.;
(b) stripping the activated carbon of (a) with either (i) a liquid organic compound selected from the group consisting of ketones, ethers, decalin, tetrahydrofurans, sulpholanes, glymes and formamides and which is non-reactive with sulphur trioxide or sulphuric acid, or (ii) a liquid organic compound capable of forming organic sulphates or sulphonates by reaction with sulphur trioxide or sulphuric acid, and
(c) recovering the products so obtained.
In a preferred embodiment of the present invention, the product is recovered by separating the non-reactive liquid organic compound of (i) or unreacted liquid organic compound of (ii) by flashing or multistage distillation.
In embodiments of the present invention, the organic compounds wet the activated carbon, preferably being imbibed into pores in the activated carbon, which is preferably in a packed bed or structured packing.
In further embodiments, the reactive organic liquid capable of forming organic sulphates or sulphonates is selected from the group of alkyl-aromatic compounds e.g. dodecylbenzene, dodecyinaphthalene or any linear or branched alkylbenzene with a chain length of 12 to 18 carbon atoms; phenols; fatty alcohols e.g. linear alcohols with a carbon chain of 12 to 18 carbon atoms; long chain olefins; and any other organic compound capable of forming organic sulphate or sulphonate compounds by reaction with sulphur trioxide or sulphuric acid.
In yet another embodiment, the product obtained is highly concentrated sulphuric acid.
In a still further embodiment, the product obtained is an organic sulphate or sulphonate.
In other embodiments, the sulphur trioxide is obtained by oxidation of sulphur dioxide, the sulphur dioxide being obtained by burning of sulphur, from flue gas or from an industrial or natural waste stream.
In a preferred embodiment, step (a) is carried out in an atmosphere of a compound or element selected from the group consisting of carbon dioxide, ethane, propane, nitrous oxide, xenon, trifluoromethane, low molecular weight fluorocarbon and fluoroethane under near critical or supercritical conditions, especially in which step (a) is additionally carried out in the presence of a modifier compound selected from the group consisting of ketones, ethers, decalin, tetrahydrofurans, sulpholanes, glymes and formamides.